1. Field of the Invention
The present invention relates in general to storage systems, and in particular to controlling multi-tiered storage systems.
2. Description of the Related Art
Multi-tiered storage systems typically include multiple logical and physical storage devices. These storage devices are divided into multiple levels, wherein each level is assigned a priority. Data is stored in particular ones of the storage devices, based on the expected demand for a particular set of the data and a desired level of performance from the storage devices.
In an example, a multi-tiered storage system may be a three-tier system. In such case, a plurality of storage devices having very high performance capabilities (e.g., for frequent data access and minimal access delay) is employed in a highest level of the tiered storage system. This tier of the tiered storage system is sometimes referred to as the “online” tier or T0 and typically consists of storage devices which are the most expensive to manufacture and to purchase. A middle tier of the tiered storage system, sometimes referred to as the “nearline” tier or T1, has storage devices having a lower performance capability than T0 but that perform sufficiently to handle regularly accessed data or data capable of tolerating larger access delays due to lower performance measures of the storage devices. There may be multiple middle tiers in a tiered storage system based on the complexity of the tiered storage system and the differing performance capabilities of the storage devices employed. A bottom tier of the tiered storage system, sometimes referred to as the “offline” tier, may be comprised of relatively low performance storage devices. This tier archives or stores data that is infrequently accessed and thus, the access delay associated with these storage devices is not of a concern.